The present invention relates to a digital frequency display device for use in communication equipment. In modern communication equipment such as radio receivers, transmitters and transceivers, a frequency display device is frequently used to digitally display the RF operating frequency. A conventional digital display device for a single conversion type receiver, for example, is constructed so that the output signals of the local oscillator and BFO of the receiver are mixed in a frequency mixer to produce a signal corresponding to the RF operating frequency, and that signal is in turn applied through a frequency counter to a digital display circuit for visual display. A similar approach is used for conventional display devices for double conversion type receivers. In such cases, the outputs of the heterodyne oscillation the VFO and the BFO are mixed to produce a signal corresponding to the RF operating frequency. Problems exist with these types of devices, however, because the display frequency produced through the frequency mixing is the same as that of the signal being received, as a result of which, signal interference can occur in the receiving circuit. Also, with such a system, leakage signals from a transmitter can disrupt proper display operation of the display device. This problem requires strict shielding for the communication equipment, resulting in increased cost.
To avoid these problems, it is proposed that a local oscillator be provided separately from the receiving or transmitting circuit, and that its output be applied to a mixing circuit for mixing with the output of the conventional local oscillator or of the VFO of the communications apparatus. With such a system, the resulting frequency is different or shifted from the RF operating frequency. However, if display is only made for cetain of the digits provided by the counter, for example, if the Mhz digits are not displayed, but the KHz digits are displayed, the above-mentioned problem can be overcome.
Also, with conventional systems, sideband switching of the carrier/BFO frequency or band switching generally causes the locally generated frequencies to be changed. These changes make it difficult in conventional systems to provide a correct display of the actual RF operating frequency. Conventionally, a frequency correction knob for the local oscillator or a preset trimmer for the heterodyne oscillator is provided for each frequency band to make the necessary deviation corrections. These approaches, however, lead to complex circuits with poor stability.